0:07Skip to 0 minutes and 7 secondsWILL HOMOKY: So conceptually, this bucket represents our ocean, and Rachel here is holding the rivers which fill our oceans with water. But what we can see is that the bucket is overflowing and it's losing water. And in fact, it's losing water at the same rate at which it's entering the oceans. And this is true for the oceans as a whole. We know that the volume isn't changing over time. And therefore, the system is said to be in steady state.

0:30Skip to 0 minutes and 30 secondsPROFESSOR RACHEL MILLS: So we know our bucket here is 10 litres and the water is flowing in at about a litre a minute. So we can calculate the average time a water molecule will spend in the bucket. It's the volume of the bucket 10 divided by about a litre a minute. So the water spends 10 minutes in the bucket before it overflows over the side. And that's what we call the residence time.

Calculating residence time

In this video, Rachel and Will give a more practical demonstration of residence time.

As was explained in the video ‘Where does the salt come from?’, the concept of an ‘average’ river has no real physical meaning but it is useful for an exercise where we compare the average river input to the overall salt content of the ocean.

[Advanced] The download CALCULATING RESIDENCE TIME below provides the information you need to calculate the residence time of the major salt constituents of seawater so you can see how those times compare.